Contact hole structure and method of fabricating the same

ABSTRACT

A method of fabricating a contact hole structure includes providing a substrate with an epitaxial layer embedded therein. Next, an interlayer dielectric is formed to cover the substrate. After that, a first hole is formed in the interlayer dielectric and the epitaxial layer. Later, a mask layer is formed to cover a sidewall of the first hole and expose a bottom of the first hole. Subsequently, a second hole is formed by etching the epitaxial layer at the bottom of the first hole and taking the mask layer and the interlayer dielectric as a mask, wherein the first hole and the second hole form a contact hole. Then, the mask layer is removed. Finally, a silicide layer is formed to cover the contact hole.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation application of and claimspriority to U.S. patent application Ser. No. 15/726,358, filed on Oct.5, 2017, and entitled “CONTACT HOLE STRUCTURE AND METHOD OF FABRICATINGTHE SAME” the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a contact hole structure, and moreparticularly to a contact hole in the shape of a pacifier nipple.

2. Description of the Prior Art

Integrated circuits (ICs) are fabricated on wafers. Commonly, thesewafers are semiconductor materials; for example, silicon. Research anddevelopment has seen a decrease in the size of the transistors making upthe integrated circuits.

As transistor sizes shrink, the contact area between the source anddrain regions and a metal contact plug for communicating with thetransistor decreases proportionally. Contact resistance of the sourceand drain regions increases proportionally with the decrease in thecontact area. Thus, smaller contact areas lead to higher contactresistances. The contact resistance is parasitic and degradesperformance of the MOSFET. A portion of the contact resistance resultsfrom a Schottky barrier between the metal contact plug and thesource/drain regions.

SUMMARY OF THE INVENTION

Accordingly, there is a need for an improved fabrication process andstructure that lowers the Schottky barrier without compromising the areaof the source and drain regions, in order to improve device performance.

According to a preferred embodiment of the present invention, a contacthole structure comprises a substrate. An epitaxial layer is embeddedwithin the substrate. A contact hole is disposed in the epitaxial layer,wherein the contact hole is in the shape of a nipple of a pacifier. Asilicide layer fills in the contact hole.

According to another preferred embodiment of the present invention, amethod of fabricating a contact hole structure comprises providing asubstrate with an epitaxial layer embedded therein. Next, an interlayerdielectric is formed to cover the substrate. After that, a first hole isformed in the interlayer dielectric and the epitaxial layer. Later, amask layer is formed to cover a sidewall of the first hole and expose abottom of the first hole. Subsequently, a second hole is formed byetching the epitaxial layer at the bottom of the first hole and takingthe mask layer and the interlayer dielectric as a mask, wherein thefirst hole and the second hole form a contact hole. Then, the mask layeris removed. Finally, a silicide layer is formed to cover the contacthole.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 to FIG. 7 depict a method of method of fabricating a contact holestructure according to a preferred embodiment of the present invention,wherein:

FIG. 2 is a fabricating stage following FIG. 1;

FIG. 3 is a fabricating stage following FIG. 2;

FIG. 4 is a fabricating stage following FIG. 3;

FIG. 5 is a fabricating stage following FIG. 4;

FIG. 6 is a fabricating stage following FIG. 5; and

FIG. 7 is a fabricating stage following FIG. 6.

FIG. 8 shows an enlarged view of an outline of the contact hole withinthe epitaxial layer of FIG. 7.

DETAILED DESCRIPTION

FIG. 1 to FIG. 7 depict a method of method of fabricating a contact holestructure according to a preferred embodiment of the present invention.As shown in FIG. 1, a substrate 10 is provided. Agate structure 12 isformed on the substrate 10. Two epitaxial layers 14 are respectivelyembedded within the substrate 10 at two sides of the gate structure 12.The gate structure 12 includes a gate electrode 16, and a gatedielectric layer 18. A cap 20 may cover the gate structure 12. Aninterlayer dielectric 22 covers the substrate 10. The substrate 10 ofthe present invention may be a bulk silicon substrate, a germaniumsubstrate, a gallium arsenide substrate, a silicon germanium substrate,an indium phosphide substrate, a gallium nitride substrate, a siliconcarbide substrate, or a silicon on insulator (SOI) substrate.

The gate electrode 16 may be doped polysilicon or metal. The gatedielectric layer 18 may be silicon oxide, silicon nitride, siliconcarbide nitride, silicon oxynitride, silicon carboxynitride, high-kdielectrics or a combination thereof. Each of the epitaxial layers 14may be a single layer formed by a stressed material such as silicongermanium (SiGe), silicon phosphorus (SiP),or silicon carbide (SiC).Each of the epitaxial layers 14 may also be a multiple layer including acap layer 24, a stress material 26 and a buffer layer 28. According to apreferred embodiment of the present invention, the buffer layer 28 issilicon, the stress material 26 is SiGe, and the cap layer 24 issilicon. The interlayer dielectric 22 may be silicon nitride, siliconoxide, silicon oxynitride or another insulating layer.

As shown in FIG. 2, the interlayer dielectric 22 and the epitaxiallayers 14 are etched to form a first hole 30 penetrating the interlayerdielectric 22 and part of the epitaxial layers 14. In this embodiment,there are two first holes 30 respectively on the two epitaxial layers14. The first hole 30 has a width W. As shown in FIG. 3, a mask layer 32is formed to conformally cover the interlayer dielectric 22 and thefirst holes 30. In detail, the mask layer 32 contacts a bottom 130 and asidewall 230 of each of the first holes 30. The mask layer 32 may be acarbon-containing layer or other material which has a different etchratio from the interlayer dielectric 22. According to a preferredembodiment of the present invention, the mask layer 32 may behydrocarbon. Furthermore, a thickness T of the mask layer 22 is about ¼of the width W.

As shown in FIG. 4, the mask layer 32 is anisotropically etched toremove the mask layer 32 on the bottom 130 of the first hole 30 and onthe top surface of the interlayer dielectric 22. The remaining masklayer 32 will only be on the sidewall 230 of the first hole 30. Theanisotropical etching process may be a dry etching using oxygen plasma.As shown in FIG. 5, a second hole 34 is formed by etching the substrate10 at the bottom 130 of the first hole 30 and taking the remaining masklayer 32 and the interlayer dielectric 22 as a mask. The first hole 30and the second hole 34 form a contact hole 36.

As shown in FIG. 6, the mask layer 32 is removed entirely. It isnoteworthy that the contact hole 36 at the epitaxial layer 14 is in theshape of a step. As shown in FIG. 7, a barrier layer 38 is formed in thecontact hole 36. Then, a conductive layer 40 fills in the contact hole36. The barrier layer 38 may be titanium (Ti), titanium nitride (TiN),titanium tungsten nitride (TiWN) or tungsten nitride (WN). Theconductive layer 40 may be tungsten (W), aluminum (Al) or other metals.According to a preferred embodiment of the present invention, thebarrier layer 38 is TiN/Ti, and the conductive layer is W. After that,the barrier layer 38 is heated to transform part of the barrier layer 38into a silicide layer 42. In this embodiment, the silicide layer 42 istitanium silicide (TiSi). During the process of transforming the barrierlayer 38, part of the substrate 10 is consumed to be transformed intothe silicide layer 42. Therefore, the step-shaped contact hole 36resembles a nipple of a pacifier. At this point, a contact holestructure 50 of the present invention is completed.

Please refer to FIG. 7. According to a preferred embodiment of thepresent invention, a contact hole structure 50, includes a substrate 10.The substrate 10 has a top surface 11. An epitaxial layer 14 is embeddedin the substrate 10. An interlayer dielectric 22 covers the substrate. Acontact hole 36 is disposed in the interlayer dielectric 22 and part ofthe epitaxial layer 14. It is noteworthy that the contact hole 36includes the shape of a pacifier nipple. A silicide layer 42 such asTiSi is disposed in the contact hole 36 in the epitaxial layer 14. Thesilicide layer 42 contacts the entire inner surface of the contact hole36. A barrier layer 38 such as TiN/Ti is disposed in the contact hole36. The conductive layer 40 such as W is also disposed in the contacthole 36. The conductive layer 40 in the contact hole 36 within theepitaxial layer 14 is also in the shape of a pacifier nipple. Theepitaxial layer 14 may be a multiple layer including a buffer layer 28such as silicon, a stress material 26 such as SiGe, and a cap layer 24such as silicon.

A gate structure 12 is disposed on the substrate 10. The gate structure12 is at a side of the epitaxial layer 14. The gate structure 12includes a gate electrode 16, and a gate dielectric layer 18. A cap 20may cover the gate structure 12. The gate electrode 16 may be dopedpolysilicon or metal. The gate dielectric layer 18 may be silicon oxide,silicon nitride, silicon carbide nitride, silicon oxynitride, siliconcarboxynitride, high-k dielectrics or any combination thereof.

FIG. 8 shows an enlarged view of an outline of the contact hole withinthe epitaxial layer of FIG. 7. As shown in FIG. 8, the contact hole 36includes a nipple bottom 52 and a nipple tip 54. Furthermore, the nipplebottom 52 has an opening 37 which is coplanar with the top surface 11,and a first width A of the opening 37 is larger than a third width E ofany part of the contact hole 36 below the opening 37, and the firstwidth A and the third width E are parallel to the top surface 11. Thefirst width A is greater than a second width B of the nipple tip 54, anda ratio of the first width A to the second width B is between 1.4 and3.3. Furthermore, the nipple bottom 52 has a first length C, the nippletip 54 has a second length D, and a ratio of the first length C to thesecond length D is between 0.17 and 0.4. Furthermore, an inner surfaceof the contact hole 36 has a convex curved portion 60 and a concavecurved portion 62, the convex curved portion 60 is in a shape of a firstcircular arc and the concave curved portion 62 is in a shape of a secondcircular arc. Furthermore, the convex curved portion 60 and the concavecurved portion 62 directly contact the silicide layer 42.

The present invention utilizes two etching process to form the contacthole and, via the mask layer, makes the contact hole step-shaped. Afterforming the barrier layer and the conductive layer, the shape of thecontact hole in the epitaxial layer resembles a nipple of a pacifier.Therefore, the conductive layer in the contact hole will also be in theshape of a pacifier nipple. Due to the special shape of the conductivelayer, the conductive layer has a large contact area between theepitaxial layer. In this way, the Schottky barrier between theconductive layer and the epitaxial layer is decreased, and theresistance between the conductive layer and the epitaxial layer can bereduced. Furthermore, even though some of the epitaxial layer is lost toform the contact hole, the epitaxial layer still retains enough area toserve as source/drain regions for the gate structure.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. A contact hole structure, comprising: a substratehaving a top surface; an epitaxial layer embedded within the substrate;a contact hole disposed in the epitaxial layer, wherein the contact holein the epitaxial layer has an inner surface comprising a convex curvedportion and a concave curved portion, the convex curved portion is in ashape of a first circular arc and the concave curved portion is in ashape of a second circular arc, and wherein the contact hole has anintersection coplanar with the top surface, and a first width of theintersection is larger than a third width of any part of the contacthole below the intersection, and the first width and the third width areparallel; and a silicide layer filling in the contact hole, wherein theconvex curved portion and the concave curved portion directly contactthe silicide layer.
 2. The contact hole structure of claim 1, furthercomprising: an interlayer dielectric covering the substrate and theepitaxial layer, wherein the contact hole extends into the interlayerdielectric; and a gate structure disposed on the substrate at a side ofthe epitaxial layer.
 3. The contact hole structure of claim 1, furthercomprising: a conductive layer filling in the contact hole.
 4. Thecontact hole structure of claim 1, wherein the contact hole furthercomprises a tip, the first width is greater than a second width of thetip, and a ratio of the first width to the second width is between 1.4and 3.3.
 5. The contact hole structure of claim 1, wherein the innersurface has a positive curvature and a negative curvature.
 6. Thecontact hole structure of claim 1, wherein the silicide layer directlycontacts an entirety of the inner surface of the contact hole in theepitaxial layer.
 7. A contact hole structure, comprising: a substratehaving a top surface; an epitaxial layer embedded within the substrate;a contact hole disposed in the epitaxial layer, wherein the contact holein the epitaxial layer has an inner surface comprising a convex curvedportion and a concave curved portion, the convex curved portion is in ashape of a first circular arc and the concave curved portion is in ashape of a second circular arc, and wherein the contact hole has anintersection, and the intersection has an opening which is coplanar withthe top surface of the substrate, and a first width of the intersectionof the contact hole is larger than a third width of any part of thecontact hole below the intersection thereof, and the first width and thethird width are parallel to the top surface of the substrate, whereinthe contact hole further has a portion thereof shaped as a nipple tipand a nipple bottom, the nipple tip is disposed below the nipple bottomwithin the contact hole, a width of the nipple bottom is greater than awidth of the nipple tip; and a silicide layer filling in the contacthole, wherein the convex curved portion and the concave curved portiondirectly contact the silicide layer.
 8. The contact hole structure ofclaim 7, wherein the concave curved portion in the shape of the secondcircular arc is disposed in the nipple bottom and the convex curvedportion in the shape of the first circular arc is disposed at atransition of the nipple bottom and the nipple tip of the contact hole.9. The contact hole structure of claim 7, wherein the nipple bottom hasa first length, the nipple tip has a second length, and a ratio of thefirst length to the second length is between 0.17 and 0.4.
 10. Thecontact hole structure of claim 7, wherein a ratio of the width of thenipple bottom to the width of the nipple tip is between 1.4 and 3.3.